It is a well known fact that elastomeric materials may contract substantially, and become much stiffer at low temperatures. These properties often make elastomeric seals unsuitable for use in cold environments, especially in low pressure sealing conditions with gas, and may in the worst case result in leakage. The consequences of spillage of hydrocarbons or aggressive chemicals may be serious. In addition to potential personnel damage, or even loss of life, damage to the environment may also occur. The last point is highly relevant in cold areas, wherein the environment is very sensitive to pollution.
There are a number of elastomeric materials for sealing at very low temperatures; however most of them are hardly compatible with aggressive fluid service or typically are not able to maintain the required high temperature exposure. A well known example is the reduction of nitrile content in NBR elastomer compounds. Such reduction leads to a substantially increased cold performance, but has a simultaneous negative impact on chemical resistance.
Non-metallic seals are also formed from elastomeric compounds that typically enclose medium or large volumes of various fillers to impart higher hardness and strength, enlarge lifetime at high temperatures and enhance resistance to various forms of material degradation. These fillers may constitute silica, carbon black, graphite, graphene, carbon nanotubes, nanoclays or a specific combination thereof, as for example is disclosed in U.S. Patent Application Publication No. 2005/0161212 A1; U.S. Pat. Nos. 7,604,049, 8,096,353, 8,314,177 and 7,696,275; and International Patent Application Publication No. WO 2005/014708 A1. Unfortunately, adding those reinforcement fillers into elastomer formulations does not give any benefit for low temperature flexibility and resilience of the elastomeric seals, and very often inhibits the latter, which manifests itself in an increase of minimum leakage temperatures.
Therefore other approaches need to be considered for sealing aggressive media under cold conditions.
U.S. Pat. No. 5,294,133 describes a solution for sealing two concentric elements at low temperatures. Basically, this patent describes a hollow elastomeric O-ring filled with a liquid. The fluid core expands in cold weather with temperature drop and compensates for thermal contraction and loss of resilience of the elastomer. The solution proposed requires injection of the liquid by a syringe or a needle through the O-ring wall, which is detrimental for seal integrity. Another drawback of the patent is a low temperature limit associated with the liquid freezing point.
WO 94/03743 discloses a seal with low thermal expansion. The seal comprises a polymer body defining a cavity which may be filled with a material that expands as the temperature decrease, i.e. a material having a negative CTE. The filling material may consist of water, hydrogel, water swellable polymers or a combination thereof. A drawback of such seals is that the main elastomer body has an opening for placing the fill material which needs to be sealed after filling the internal cavity with the filler material. This is not a preferred solution for high pressure retaining systems such as oil and gas producing equipment, since seal integrity is compromised. An alternative solution involving filler injection through a syringe or needle into the internal space of the seal has also been proposed (which is analogous to U.S. Pat. No. 5,294,133, and with the same integrity issue).
It is also known to use a seal that is larger than the volume where it is located so that when it contracts it is still able to fill the volume. However, such a seal has a large drawback in that it is prone to extrusion out of its volume (e.g. out of its groove).
The present invention aims to provide seals which alleviate or avoid at least some of the known drawbacks and disadvantages of the prior art techniques.